1. Field of the Invention
This invention relates generally to motorcycles, and more particularly to motorcycle suspension systems.
2. Description of the Prior Art
Harley-Davidson is a well known motorcycle manufacturer that manufactures a popular suspension system for its motorcycles under the trademark SOFTAIL. The SOFTAIL.RTM. suspension has no visible suspension components and thus mimics the desirable appearance of the Harley-Davidson "hard tail" frame in which the connecting arms of the rear wheel are rigidly fixed to the frame. The SOFTAIL.RTM. suspension system incorporates a swing arm and shock absorbers which dampens the effect of the rear wheel attachment, thereby affording an advantage over the "hard tail" frame by providing some suspension cushioning that improves the handling and riding comfort of the motorcycle.
In the SOFTAIL.RTM. suspension system, the swing arm pivotally attaches to the struts of the motorcycle main frame, thus enabling the rear wheel to arcuately move independently of the frame. The swing arm includes a pair of wheel mounts located on a wheel axis spaced to receive the rear wheel. A pair of arms extending from each of the wheel mounts includes a connector connecting each pair of arms. Each connector is joined to the other connector by an upper transverse cross member and a lower transverse cross member. An intermediate transverse cross member extends through the connectors and through the frame struts of the motorcycle main frame. The swing arm is pivotal about this intermediate transverse cross member. The connectors have the same general size and shape as the frame struts of the main frame. Each connector is positioned adjacent and inside the corresponding frame struts so they are hidden from view.
The standard or stock SOFTAIL.RTM. suspension system uses two shock absorbers mounted in parallel relationship with the motorcycle main frame. One end of each shock absorber is attached to the main frame and the other end is connected to a projection extending forwardly from the lower transverse cross member of the swing arm. The projection is adapted to receive a bolt or pin which connects the shock absorber to the projection. The main frame of the motorcycle hides the projection and shock absorbers from view, thereby retaining the aesthetic appearance of the desirable Harley-Davidson "hard tail" frame.
As mentioned previously, the SOFTAIL.RTM. suspension system affords an advantages over the "hard tail" frame by providing some suspension cushioning that improves the handling and riding comfort of the motorcycle. However, as discussed in U.S. Pat. No. 5,487,443 (hereinafter the '443 patent), significant disadvantages remain. For example, the ride remains too harsh and uncomfortable because of the very short travel of the shock absorbers.
As mentioned in the '443 patent, the maximum vertical movement of the rear wheel relative to the main frame is limited to two inches in either vertical direction before the swing arm contacts the main frame. The shock absorbers, have an even smaller maximum travel of 1.5 inches, 0.75 inches in either direction from a center position. The result is that while the rear wheel may travel two inches in one direction from its centered position, the shock absorbers can only travel a distance of 0.75 inches. This limited distance of travel of the shock absorber results in a hard, uncomfortable ride because all of the shock absorption must occur within this narrow range of movement.
Further, the short distance of travel mandates continual and rapid changes of the shock absorber shaft according to the movement of the wheel. Often, these directional changes in the shock absorber cannot occur as fast as necessary to create a smoother ride due to the inherent reluctance of the shock absorber to rapidly change directions. As discussed in the '443 patent, this inherent reluctance to rapid directional change is due to the relatively large momentum forces involved, the inertial drag caused by the movement of the oil or gas within the shock absorber, and the friction resistance caused by the seals and o-rings.
There have been previous attempts to improve the SOFTAIL.RTM. suspension system, but these attempts have had little success. For example, it has been attempted to increase the travel distance of the shock absorbers. This solution, however, offers only a modest improvement to the ride of the motorcycle because it does nothing for the responsiveness of the shock absorbers. Additionally, the range of movement of the swing arm is limited by the motorcycle frame, and therefore, the amount of travel of the shock absorber is also limited. Any further increases in the shock absorber travel requires modification to the motorcycle main frame, which is very undesirable. Other attempts have been made to improve the responsiveness of the shock absorbers, however, these attempts are hindered by the drag and friction forces discussed above.
In addition to the problems associated with the ride and handling of the standard or stock SOFTAIL.RTM. suspension, many motorcycle enthusiasts attempt to lower the rear of the motorcycle for aesthetic reasons, believing it improves the appearance of the motorcycle. Special kits are sold to allow the frame to be lowered, however, the lower of the rear of the motorcycle reduces the stroke on the suspension as discussed above resulting in a rougher ride.
Therefore, there remains a need for a motorcycle suspension system that maintains the aesthetic appearance of the SOFTAIL.RTM. frame while improving the ride and comfort of the motorcycle. There also remains a need for a suspension system that enables the rear of the motorcycle to be raised and lowered when desired for aesthetic purposes.